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Microraptor leg feathers and the evolution of bird flight

A recent abstract by Habib et al. 2012 hypothesized that “Microraptoran dinosaurs may have experienced intrinsic difficulties with pitch control because they retained a trunk of typical dromaeosaurid proportions, as opposed to the shortened trunk of ornithurine birds.

Figure 1. Microraptor with feather impressions.

“Specimens of Microraptor gui show that a fan of feathers existed near the terminus of the tail. This would be sufficient to correct for small deviations of the center of gravity from the center of lift. The tail could not have provided significant control in yaw or roll, but the forewings and hindwings would have been well suited to providing those control functions.

Figure 2. Microraptor standing. Here you have wings, a horizontal stabilizer on the tail and vertical stabilizers on the hind legs. With such small wings and inexperience as a pilot, you need all the control surfaces you can get!

“We suggest that a new and more compelling general model for the evolution of flight in paravians and early birds is emerging. Early in the evolution of theropod flight, major flight control functions were relatively evenly distributed between the forewings and the auxiliary control surfaces – namely, the hindwings and tail. This allowed the comparatively robust hindlimbs and tail of paravians to carry much of the mechanical loading associated with tight maneuvers, launching, and landing. In more derived members of the avian line, most control function shifted to the forewings, though primary launch power continued to be provided by the hindlimbs. This model explains how animals such as Microraptor could fly in cluttered environments with small pectoral muscle fractions and gracile forelimbs.”

I did not see the presentation by Habib et al. 2012. Funny that the abstract title (see below) focuses on the tail, when the hind limb feathers are what everyone is chatting about. Their presentation made some news here at Findognews.blogspot.com. The opening paragraph states, “A rethink of four-winged dinosaurs suggests that the much-debated hind wings stayed tucked under the body until deployed in the air for tight turns to dodge branches or chase prey.”I suppose this was the gist of the talk.

ScientificAmerican.com included this statement by Habib, “A combination of pitch control by the tail, roll generation by the ‘hindwings’ and multi-purpose control by the main wings would have madeMicroraptor a highly maneuverable animal.”Seems more than reasonable.

The blogspot went on with history of the hind leg feathers, “The first reconstruction showed the small dinosaur gliding in the air with all four limbs extended outward. A later proposal lowered the hind-limb feathers for a Wright-Brothers biplane of wings. Both arrangements have drawn criticism. In a simpler solution, the dinosaur could have kept its hind limbs under its body much of the time until needed for banking in a turn.”

Figure 3. Early illustration of Microraptor sprawling like a flying lizard, not a flying dinosaur.

Yes, that first sprawling Microraptor illustration (Fig. 3) had everyone aching, as it essentially popped those dinosaurian right angle femora out of their sockets.

Kevin Padian commented about the abstract in the blogspot, “Powered flight and gliding downward have developed in quite different evolutionary branches.” Maneuverability is certainly important to both, but he does not see gliding as an evolutionary baby step on a path toward powered flight.”

Dr. Padian jumped on the gliding half of the hypothesis because he observed that the presentations focused on the effect of the hindlimb on a gliding animal. From the Scientific American blogspot, “He questioned why the team’s model would focus on gliding parameters when the forelimb shape was consistent with flapping, not gliding, and the hindlimb would have generated so much drag.”

Not sure about hind feather drag.Cylinders create lots of drag. Bare legs are cylinders. Feathered legs turn those cylinders into tear drop shapes, which minimize drag while providing large surfaces that can be turned into the line of flight in order to increase drag, redirect the airstream and maneuver, the way airplanes do. That’s why wheels on airplanes are often given “pants.” That’s why vertical stabilizers are shaped like they are.

The coracoids of Microraptor were shaped and immobilized in the manner or birds and fenestrasaurs (including pterosaurs), which are ideal for for flapping. Maybe not great flapping, but you got start somewhere.

Figure 4. Longisquama in lateral view, dorsal view and closeup of the skull. Like Microraptor, Longisquama glided/flew with similarly-sized wings both fore and aft, and here with a much longer torso, ideal for leaping. Note the stem-like coracoids, a sure sign of a flapper.

The distribution of flight control systems across all four limbs(and tail) find an interesting parallel in Longisquama (Fig. 4), a sister to the Pterosauria, in which the forelimb wings and hind wing uroptagia formed four wings. Here again, the forelimbs could have provided thrust while in the air. In the air the hind limbs could have acted like vertical stabilizers, or, when extended laterally, could have provided lift, but not flapping.

While the key reason for developing hind leg flight surfaces might have been for maneuverability, the most important moment in that maneuverability would have been a two-point landing following a positive pitch flare, bringing the airspeed down to zero while still maintaining control. I think Habib et al. (2012) are right on the money. There are analogous reptiles demonstrating the same sort of evolution (Longisquama) and it all makes sense in every way. If they didn’t talk enough about flapping, that’s a minor point.

And those extra feathers ain’t a bad secondary sexual character either.

As always, I encourage readers to see specimens, make observations and come to your own conclusions. Test. Test. And test again.

Evidence and support in the form of nexus, pdf and jpeg files will be sent to all who request additional data.

2 thoughts on “Microraptor leg feathers and the evolution of bird flight”

I thought at the time that Padian missed the point of the talk which was not about powered vs. unpowered flight but how to control the animal when it was moving forward in the air. The control surfaces are still needed so this hypothesis about their structure and functions are equally applicable.

The bottom line, IMO, is that these guys became flyers because of their flapping forelimbs, which must have had a prior “cause”. The other features are secondary to that: (1) Flapping forelimbs, (2) powered flight, and only then (3) secondary mods to facilitate flight (such as feathered hind limbs and tail). Also, attributing VERY significant features such as flapping forelimbs-becoming-wings to sexual selection is, IMO, a cop out because it means that such features can’t be explained in terms of natural selection and adaptation to environment, which I don’t think is the case.